The present invention is directed towards a transformer differential relay. More particularly, the present invention is directed towards a transformer differential relay which monitors the current flowing through the primary and secondary windings of a power transformer and disconnects transformer from a power source feeding the transformer whenever the magnitudes of these currents indicate a fault condition in the transformer. A fault condition is detected whenever the magnitude of an operate signal (representative of the magnitude of the vector sum of the primary and secondary winding currents) reaches a first predetermined percentage of a restraint signal (representative of the magnitude of the larger of the primary and secondary winding currents).
While transformer differential relays of the foregoing type are generally known, these relays utilized a plurality of expensive and bulky voltage transformers to generate the restraint and operate signals. Typically, the restraint signal is generated by a pair of series-connected restraint transformers whose primary windings receive primary and secondary current signals of opposite polarity and respectively proportional in magnitude to the currents in the primary and secondary windings of the power transformer being monitored. These current signals are derived via a pair of current transformers connected in series with the primary and secondary windings of the power transformer. The secondary windings of each of the restraint transformers are connected to full-wave diode bridges whose outputs are connected in parallel so as to generate a restraint signal representative of the larger of the secondary and primary winding currents.
The operate signal is typically generated by an operate transformer whose primary winding is coupled between an intermediate tap of the primary windings of the two restraint transformers and ground so as to form a T-connection therewith. As such, the primary winding of the operate transformer receives the primary and secondary current signals in an additive manner. The secondary winding of the operate transformer is connected to a full-wave diode bridge whose output represents the operate signal. The operate and restraint signals are then compared by a polarized relay which derives a percentage differential characteristic. When the operate current rises above the predetermined percentage of the restraint current, the relay disconnects the power transformer from the power source.
The use of restraint and operate transformers in the foregoing prior art differential relay requires a complex interconnection of tap blocks to conserve the necessary T-connection. Additionally, the use of conventional bridge diode rectifiers results in at least a fixed voltage drop across the rectifiers. As a result, the magnitude of the restraint and operate signals must be sufficiently high to make this fixed voltage drop insignificant. This leads to a large signal operation and adversely affects the size of the transformers. Accordingly, the prior art differential relays are bulky, heavy and require complex tap block interconnections.